So I was playing in my hardcore world, where I crafted Mining Laser for the first time. I was trying out its different modes, and when I picked "Explosive", I fired a single beam at some distance (0000005:0000006-7 blocks), and it killed me instantly (RIP my world), even though I was wearing a full iron armor set.
After that, I decided to test the damage of Explosive mode in my creative world. Results:
- it kills every mob, like enderman or even iron golem, in 1 shot (however, Wither boss takes 2 shots, and Ender Dragon cannot be damaged by Explosive mode)
- at point blank, it kills a player in full Nanosuit armor set in 1 shot
- Quantumsuit can take about 3 shots at point blank, the next shot will kill you
- your distance from the explosion does matter, but anything weaker than Nanosuit will most likely die even at the edge of explosion (but that's not the case with mobs, they seem to receive much less damage at the edge)

Activities

This is the first time I have seen someone saying the damage is too high, iirc.
Anyway, I tested the damage from 1 block away (by going to survival mode without wearing armor and using the health_boost effect to see how much damage I took without killing me), explosion radius (destroying a square of slime block) and explosion results on stone. Here are the results:
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Damage
TNT: 22 hearts
Industrial TNT: 60 hearts
Mining laser:
1 block: 168 hearts
2 block: 132 hearts
3 block: 96 hearts
4 block: 72 hearts full set iron armor
5 block: 36 hearts 31.5 hearts
6 block: 24 hearts 21 hearts
7 block: 24 hearts 21 hearts
8 block: 12 hearts
9 block: no damage
---
Explosion Radius (on slime)
TNT: 4 blocks, random on the edge
Industrial TNT: 2 blocks
Mining laser: 4 blocks, with a ring 1 block wide
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Explosion result on stone
TNT: irregular shape, different every time
Industrial TNT: irregular shape, same every time, took out 34 blocks
Mining laser: regular shape, same every time, took out 37 blocks
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So... The results clearly show that it is deadly when being too close to the explosion even with good armor. The damage of the laser is somewhat too high comparatively and obviously unbalanced when using it to finish a wither in 2 sec. The explosion damage should be set to a lower value in close range or add a config option for it. The explosion damage on blocks is quite balanced.

The damage is specifically specified to be more than iTNT in the code, as well as the actual block damage to be a tad weaker. 168 hearts is quite the impact though, might be worth going back to see if it has varied between Minecraft versions that dramatically.

Did further testing with a range of 1 block. It seems that the damage is different when hitting different sides of a block despite the same range.
Results:
1.7.10: 132 hearts
1.8.9: 144 hearts
1.9.4: 132 hearts
1.10.2: 156 hearts
1.11.2: 156 hearts

Strange how it has drifted around when the code is effectively unchanged between the versions. I was thinking maybe the armour protection values changing might have influenced it but that wouldn't explain doing more damage without any.